Wireless telecommunications systems are known. In a cellular system radio coverage is provided in areas known as cells. A base station is located in each cell to provide radio coverage. Traditional base stations provide coverage in relatively large geographical areas and the cells are often referred to as macro cells.
A typical base station comprises: a control centre at ground level, a radio mast and an array of antenna located on the mast. The array of antenna operates to provide radio coverage to end users within the cell by transmitting signals to, and receiving signals from, end users within the cell. In such a typical base station the base station control centre contains, amongst other things: a data management unit, a digital to analogue converter, a filter and a power amplifier. Housing those items in the control centre at ground level allows those components of the system to be substantially protected from prevailing environmental conditions, such as precipitation or temperature fluctuation.
Analogue radio frequency (RF) signals generated in the control centre at ground level are communicated to each antenna element making up the antenna array via coaxial cable extending from the control centre at the base of the radio mast to the antenna array provided near the top of the radio mast. In a “passive” array the RF signal supplied to each antenna element is substantially identical; provision may be made in the antenna array for a fixed phase shift across the array to be introduced to the signal to be transmitted by the array. Such a phase shift across the array allows redirection of wave fronts produced by the array, also known as “beam forming”. An array which performs such a fixed phase shift, irrespective of signal, may be known as a “passive” system. That phase shift may be introduced by a physical tilt or other appropriate arrangement of each of the antenna elements comprising the array.
A passive antenna array introduced a fixed phase difference across an antenna array to all radio frequency signals conveyed by that array to an end user. In contrast, an “active” antenna array is able to introduce a phase shift across the antenna array appropriate to the location one or more end users receiving a signal. An active antenna array is able to simultaneously shift different signals for different end users by introducing different phase shifts. In particular, an active antenna array is able to make use of linear superposition of radio waves to transmit, for example, a high power signal to an end user a large distance from the array and a lower power signal to an end user closer to the array simultaneously by introducing different phase shifts across the array for each of those signals.
An active antenna array can thus be understood to function as a set of several parallel lower power transceiver chains which operate in a parallel manner. The parallel transceiver chains enable dynamic beam forming of transmissions to end users. It will be understood that each transceiver chain or transceiver module within such a transceiver array is directly fed with a signal according to the user equipment being served by the base station.
In any antenna array system consisting of a multitude of transceiver chains, and particularly when that array is used to generate a specific beam pattern, some degree of calibration of each transceiver chain may be necessary. Calibration ensures that the required phase and amplitude weighting of the set of signals to be transmitted to user equipment across the array can be ensured. It will be understood that, as a result of performance variations in components, temperature drift, ageing, and other variations, each transceiver chains in an array will induce a complex aberration in the signal as it passes through the chain and that each transceiver chain in an array will display non-linear behaviour. The signals at the output of each transceiver chain will vary in phase and amplitude, even if the input signals at the individual transceiver chains are identical. It is therefore required to calibrate such an array system.
It is desired to provide a calibration method for use in an improved active transceiver array.